Preparing Nursery Plants for Winter

Introduction

It usually requires more than one season to produce marketable woody nursery crops. Thus nursery crops must survive at least one winter before they can be sold. Successful winter survival means that the roots remain alive and the upper foliage is healthy, shows little damage, and is ready to resume growth.

Most plants produce a flush of growth in the spring, and this growth may continue throughout the season. Growth gradually slows in the fall because of cooler temperatures and shorter days. To grow plants more rapidly, we can extend the period of rapid growth by fertilizing, irrigating, and controlling pests; however, plants are less tolerant of low temperatures during this time.

Several physiological requirements must be met to successfully overwinter nursery plants. To choose the appropriate winter protection techniques, you must know how plants acquire cold hardiness and how they are damaged by cold temperatures and winter conditions.

Part 1: Winter Acclimation

To harden plants or induce dormancy, the proper conditions must occur at the same time. To induce winter hardiness and dormancy, interaction between photoperiod (length of daylight) and temperature are important. During overwintering, several internal processes within plants affect cell membranes, energy storage, leaf coloration, and abscision in deciduous plants. Fall cultural practices such as applying fertilizer, irrigating, pruning, digging, and controlling light can affect these internal hardening processes. Failing to provide proper conditions by neglecting any one of these factors can cause winter injury.

Fertilization

Nutritionally balanced plants have the best chance of withstanding winter conditions. If you use granular and liquid fertilizer programs that immediately supply soluble fertilizer to the plant, begin withholding fertilizer about six weeks before the average first frost date (Fig. 1). If you use slow-release fertilizers, you may have trouble reducing the plant growth late into the season. Tender growth caused by high or fluctuating rates of nitrogen fertilizer late in the season may also prevent hardening. The source of nitrogen is also important. Nitrate nitrogen is readily available to plants for uptake; however, high nitrate levels may stimulate new shoot growth.

Most information on wintering nursery crops suggest increasing potassium (K) levels to promote the cell permeability, which is important in avoiding cellular freeze damage. Although information on this practice is conflicting, maintaining adequate tissue potassium levels is advised. Foliar analysis from the North Carolina Department of Agriculture Plant and Soil Testing Laboratory indicate that potassium levels in foliage should have an index of 50 to 75 for most woody ornamentals and that a soil test index value of approximately 50 is adequate. If foliar or soil levels are well below these values, winter hardiness may be improved by applying potassium.

If you wish to apply a complete NPK fertilizer in the fall, wait until above-ground plant parts are fully dormant. After deciduous plants have dropped their leaves, a fall fertilizer application is usually safe. A moderate level of balanced fertilizer should not cause plants to break dormancy or reduce hardiness.

Watering

Either too much or too little water during the later part of the growing season can reduce the winter hardiness of nursery crops. Regular irrigation during the growing season is necessary for maximum growth and proper nutrient availability. If fertilizer has not been released during the summer because of a lack of water, it may become available during September rains, creating a flush of growth that will not acclimate before cold weather.

In the fall, reduce the frequency of irrigation for container-grown plants; however, apply enough water with each irrigation to allow some water to reach the bottom of the container. Plants subjected to very dry conditions during the fall are less able to withstand severe winter conditions than those receiving reduced irrigation even if ample water is provided during early winter.

Decreased survival is linked to reduced energy storage. Drought conditions in the fall reduce root storage. As a result, plants may not accumulate enough stored energy for bud break and shoot expansion in the spring.

When you overwinter plants in the open, you must water them occasionally. You may increase plant survival if you irrigate containers before a cold period that is expected to drop temperatures low enough to freeze the growing medium.

Using irrigation as a winter-protection technique over outdoor growing blocks is feasible only if the plants have shoot growth that has not quite hardened and temperatures are expected to drop near freezing. This technique is frequently used with peaches, apples, and strawberries in spring to protect flower buds from freezing. For nursery crops, this procedure can be used successfully in fall and spring to avoid damage to soft shoot growth. The irrigation must be applied before ambient temperatures reach 32oF and must usually be continued through several daylight hours the next day until the ice begins to melt. If discontinued sooner, freeze damage is likely to occur. However, icing in woody nursery crops also has disadvantages because the heavy coat of ice can break limbs (Fig. 2). Unprotected plants (with soft shoot growth) that suffer an early-fall or late-spring frost generally lose the current flush of growth. If soft shoot extension is 6 inches or more, you may need to prune off dead growth. Apply a fungicide in either case. The following flush usually produces multiple shoots from each shoot apex.

Pruning

Late-season pruning may stimulate bud break, resulting in new growth that does not harden off before cold weather. Avoid pruning within 6 weeks of the average first frost date. Extensive late-fall pruning also creates wounds that do not close until active growth begins in spring. This may increase the opportunity for decay organisms to become established in the wounds.

Light

Both intensity and duration of light affect plant dormancy. In the shade, plants acclimate more slowly than in the sun. For this reason, mountain growers remove shade in September to help harden plants. For example, the portion of the stem that enters the soil or potting medium is the last part of the plant to attain full winter hardiness. Early frosts may cause bark splitting in this area of the stem (Fig. 3).

Removing shade in the fall induces more rapid acclimation and decreases the potential for splitting. In piedmont and coastal nurseries, considerable growth occurs throughout the fall. Removing shade from actively growing shoots may cause sun scald on succulent shoots that are accustomed to shade. Sometimes, during a short period of time after the new growth hardens but before the extended cold arrives you may remove shade and increase hardiness. If you are going to move plants to sheltered, shaded areas, they should be fully hardened before they are moved. Mulch for winter protection only after plants are hardened by initial frosts and shorter days. Mulching may insulate the plants and reduce acclimation. In western North Carolina, both of these practices are usually performed after November 15. In other regions of the state, these steps are usually completed just before Christmas.

Turn off supplemental lighting in the fall if plants are to be wintered in unheated areas. Shorter days are just as essential as reduced fertility, irrigation, and temperature if a plant is to harden properly.

Temperature

As temperatures drop, plant growth slows and many nursery plants begin winter acclimation and dormancy. Cool temperatures and shorter days initiate the first phase of hardening, allowing plants to withstand a frost but not a hard freeze (Fig. 4).

To become fully acclimated so they can tolerate the cold associated with their hardiness zone, nursery crops require exposure to temperatures between 32°F and 40°F followed by temperatures slightly below freezing.

After plants become fully hardened, prolonged periods of warm weather can cause them to lose some degree of hardiness even if all other factors are favorable.

Hardiness Ratings

Not all plants can withstand the same degree of cold. They are usually ranked according to hardiness zone. Western North Carolina is generally ranked as Zone 6 or 7 in a normal winter, whereas the piedmont and coastal regions of the state are ranked as 7 and 8 (Fig. 5). Local conditions such as air drainage, elevation, slope, and proximity to large bodies of water can influence temperatures within a small geographical area.

Some plants, such as hybrid rhododendrons, have their own rating system:

Rating

Minimum Temperature °F

H-1

-25

H-2

-15

H-3

-5

H-4

5

H-5

15

Shoots, roots, and buds differ in their ability to withstand cold temperatures. At a given temperature, flower buds may die, whereas leaf buds remain unharmed. Roots are often damaged at higher temperatures than shoots on the same plants (Table 1). Container-grown ornamentals and plants that are not normally hardy at your nursery must be protected during the winter. Container-grown plants or plants that are not fully dormant need more protection than is indicated by a plant hardiness zone map.

Frost Burn

Damage can occur when frost forms on the leaves of evergreen plants such as hemlock, mountain laurel, azaleas, rhododendrons, camellias, osmanthus, and others. If frost covered shoots are exposed to bright sunlight, freeze damage or "burn" may occur. Foliage usually turns bright yellow in a few days because of chlorophyll degradation. This damage is usually easy to diagnose because the inner leaves (those in the shade) are not affected. There is no long-term damage from freeze bum. Once nominal growing conditions resume in the spring, leaves will return to a normal green color.

Wind Burn and Desiccation

When plants lose moisture through leaves more rapidly than the moisture can be taken up by the roots, permanent damage can occur. On broadleaved evergreens, this moisture loss results in curled leaves with dead brown tips or edges. On boxwood and conifers, foliage may turn bronze before leaf tips turn brown or black.

Drying out or winter desiccation causes more loss than freeze injury in uncovered nursery stock. Although this condition is expected in very windy locations, cold, sunny days with minimal wind can also cause severe desiccation. Wind injury is not always fatal; however, plants may not be marketable in the spring. If the soil or planting medium freezes, no moisture is available to leaves and shoots. Thus, plants can be killed to the soil line and be totally desiccated even though the temperatures would not have been low enough to kill them otherwise. In the winter, dead plants around the edge of unmulched seedbeds and transplant beds are often caused by drying out.

Figure 1. Average date of the first freezing temperature in autumn.

Figure 1. Average date of the first freezing temperature in autumn.

Figure 2. Irrigation can be used to protect succulent shoot growth only temporarily.

Figure 2. Irrigation can be used to protect succulent shoot growth only temporarily.

Figure 3. The base of the plant is the last portion of the stem to attain full winter hardiness.

Figure 3. The base of the plant is the last portion of the stem to attain full winter hardiness.

Figure 4. A typical pattern of acclimation to freezing temperatures by woody plants. Temperatures and dates vary depending on the hardiness zone and species.

Part 2: Protection Techniques

Select a site that reduces plant damage during winter months by avoiding excessive wind, frost pockets, rodent populations, and abnormally early warming in the spring or during winter thaws.

Barriers can help prevent wind bum if placed properly. Windbreaks raise the wind and reduce air movement (Fig. 6). A windbreak will protect plants on the leeward side (away from the wind) for a distance of five times its height. Poorly placed windbreaks can create swirls, causing more damage than protection. Plants that are dug and overwintered need to be placed in shade with the rootballs heavily mulched to protect the root system from freezing temperatures (Fig. 7).

Seedbeds and Liner Nurseries

Small or recently set plants usually have a reduced root system. Because of their size and small root system these plants are more likely to be "heaved" out of die ground, dry out, and die during the many freeze and thaw cycles of a normal western North Carolina winter. Fewer frost-heaving problems occur in piedmont and coastal plantings.

To safeguard against frost-heave damage, these plants are usually mulched heavily during the first winter after germinating or transplanting. Soils with a higher clay or organic-matter content seem to be more prone than sandy soils to frost heaves. Best results are obtained by mulching with 6 to 8 inches of hardwood leaves, pine needles, or clean straw after the plants are fully dormant. Covering plants with a single layer of spun-bonded polyester or polypropylene fabric has provided good winter protection as well (Fig. 8). However, heat may build up under these fabrics, causing plants to break dormancy early.

Container Nurseries

Growing plants in containers presents many special winter survival problems. The two major problems involve drying out and a lack of root hardiness. Winter desiccation, or drying out, is the most common winter injury of container grown evergreen nursery crops. If broadleaved evergreens are not watered adequately, they often turn bronze and their shoots later die. When temperatures remain below freezing for an extended period of time, the root ball can freeze completely in containers making water unavailable to the roots. To remedy the problem, irrigate adequately during the winter. When cold, sunny, or windy conditions are forecast, irrigate before the arrival of the weather front. Irrigation increases plant turgor and helps the plant move water through cell membranes. Also, if containers are irrigated to their capacity, an additional thermal resistance to freezing is provided. Plant roots are not as hardy as shoots. Roots will die at much higher temperatures than the above-ground portions of the plant (See Table 1). Plants that endure freezing temperatures in the landscape may have roots that would normally be killed at temperatures of 20°F to 25°F. In North Carolina, soil rarely drops below 20°F, except in the very upper-most soil portion. The soil provides insulation during low ambient air temperatures. Roots in unprotected containers are much more vulnerable to freezing temperatures.

Avoid laying plants over on their side for long periods of time during the winter. In light conditions, buds and shoots will turn upward and the result will be asymmetrical growth. If dormant trees are laid over and exposed to full sunlight, sunscalding on the main branches and trunk may also occur. This type of winter damage is often mistaken for mechanical injury.

A variety of winter protection techniques have been used successfully for container-grown plants. For example, in western North Carolina, nursery operators that grow only very hardy container plants may cluster them together in a sheltered, shaded location, mulching over the tops of the containers and placing bales of straw around the perimeter of the clustered pots (Fig. 9). The bales and mulch trap the air during the day, and when the temperatures drop at night, the air trapped around the containers remains warmer than the air around the tops of the plants. Shade also protects the leaves of evergreens from sun and wind, reducing water loss on bright, cold days. Spring frost burn, which occurs when the sun shines on frozen or frost covered leaves, is also prevented by shade.

In the piedmont and eastern North Carolina, container grown plants are pushed tightly together in blocks. Some nursery operators wrap the container blocks with plastic or paper to reduce air movement between containers (Fig. 10).

In recent years, many nursery operators have experimented with structureless winter protection methods. In hardiness zones 8 and 9, they have successfully protected plants by preparing them as if they were going to be placed into a structure, then laying a cover of white copolymer film or thermal blankets over the top of the plants. The sides are securely fastened and the cover checked to make sure it is unpunctured (Fig. 11). Problems with this technique have been the abrasion of plants from the plastic flapping in the wind, breakage if a heavy ice or snow storm occurs, and heat and moisture buildup under the cover. Other growers have used the structureless system, covering plants with shade cloth or fabric (Fig. 12).

Research with porous row cover fabrics indicates that they protect some nursery crops as well as if the crops were placed in winter protection structures. Shade cloth or row cover fabrics reduce sunlight and wind movement around evergreen and broadleaved evergreen plants. This reduces desiccation and discoloration of foliage, leaving greener plants with greater sales appeal for early spring marketing. During periods of bright, sunny, warm days, remove the fabrics but keep them accessible. Removal helps reduce early shoot development.

Many nursery operators build temporary "polyhouse" structures (Fig. 13, close-ups Fig. 13a, Fig. 13b, Fig. 13c, Fig. 13d, Fig. 13e, Fig. 13f, Fig. 13g, Fig. 13h). Orientation of wintering structures covered with white copolymer plastic is not as critical as with clear plastic. However, houses oriented north to south will be somewhat cooler than those facing east to west. Plants that are fully dormant or have hardened are placed in these structures, which are then covered with a plastic film. To ensure the greatest degree of hardiness, do not cover houses until the onset of extended cold winter temperatures is imminent.

Approximately six weeks to a month before covering, apply preemergence weed controls. If slow-release fertilizers were used during the growing season, conduct a test to determine the salt levels in the containers. You may use a procedure called the Virginia Tech Extraction Method (VTEM) to test the conductivity (salt level) in containers. Irrigate the containers before collecting the leachate. Approximately 5 fluid ounces (350 milliliters) of distilled water is required to obtain a leachate from a 1-gallon container; approximately 12 fluid ounces (350 milliliters) is needed for a 3-gallon container. If a VTEM pour-through leachate collection is made, salt levels of the leachate should be below 0.5 millimhos (50 Mhos). If the medium is taken from the container to do a salt test, measure 50 cc of medium and add 100 ml of distilled water. The conductivity (salt) reading should be less than 0.2 millimhos (20 Mhos). If conductivity levels are higher, leach the containers by applying approximately 1 inch of irrigation. Check the containers during the winter and do not let them become excessively dry. Random salt testing of containers in winter-protection houses may indicate theneed for further irrigation.

Shortly before they are covered, plants must be thoroughly watered and sprayed with a fungicide to prevent infection by diseases that are active at the low temperatures and high humidities found in wintering structures. Once the plant foliage has dried, the structures may be covered. The most popular covering material in North Carolina is 4- or 6- mil white copolymer plastic film. The white film provides shade, while preventing rapid temperature changes within the house. Houses covered with clear plastic film are hotter during the day and colder at night than those covered with white copolymer film. These large temperature changes are responsible for damage to plants in houses covered with clear plastic film.

Make arrangements to ventilate overwintering structures. A ventilation fan activated by a thermostat and mounted on the leeward end of the house, with louvers on the windward end, will provide the most consistent ventilation. If ventilation is provided by opening end doors, block the air movement at plant height and direct ventilation to the upper portions of the house (Fig. 14). This reduces the air movement around plants. Some growers ventilate houses by cutting progressively larger holes in the film on the sides of the houses.

Operators of nurseries located below 2,000 feet in elevation who ventilate their clear-plastic-covered greenhouses on hot days and irrigate regularly during the winter can successfully overwinter plants. Less hardy plants or colder locations may require the use of supplemental heat or greater insulation as provided by devices such as thermal blankets or inflated double-poly houses.

All nursery operators who protect plants in wintering houses must make provisions for snow and ice. Unless overwintering structures have sufficient structural strength, they may collapse during a snowstorm. (Fig. 15). Plants that have been crushed and broken by tons of snow are an expensive loss that can be prevented by adequate planning.

In spring, remove covers as early as possible so that heat buildup under the cover does not result in excessive bud swelling. However, remove covers late enough to avoid subfreezing conditions in which root and shoot damage occurs. If active shoot growth begins in enclosed structures, uncovering will cause frost damage to new shoots unless plants are left covered until after the last expected frost date. Leaf and shoot expansion under the low light conditions of white copolymer film will be wide and thin. When the film is removed, the new growth must be shaded for several weeks to prevent sunscalding. Dates for covering and uncovering vary from one location to another and from one year to the next. Growers must develop an intuitive feeling for these activities.

Figure 6. Windbreaks raise the wind and reduce air movement in a field.

Figure 6. Windbreaks raise the wind and reduce air movement in a field.

Figure 7. Rootballs of plants need to be protected from freezing by mulch.

Figure 7. Rootballs of plants need to be protected from freezing by mulch.

Figure 8. Spun-bonded polyester or polypropylene fabrics can provide good frost heave and winter protection of seed and liner beds.

Figure 8. Spun-bonded polyester or polypropylene fabrics can provide good frost heave and winter protection of seed and liner beds.

Figure 9. Container plants can be piled or pushed closely together in blocks and mulched to provide some winter protection.

Figure 9. Container plants can be piled or pushed closely together in blocks and mulched to provide some winter protection.

Figure 10. Container blocks pushed tightly together and wrapped with plastic or paper was the predominant winter protection technique before overwintering structures became popular.

Figure 10. Container blocks pushed tightly together and wrapped with plastic or paper was the predominant winter protection technique before overwintering structures became popular.

Figure 11. Structureless winter protection techniques using white copolymer film and thermal blankets reduce desiccation and provide approximately the same freeze protection given by structures.

Figure 11. Structureless winter protection techniques using white copolymer film and thermal blankets reduce desiccation and provide approximately the same freeze protection given by structures.

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